Lens f numbers

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Keith Butler

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Hi,
Can you help me with the understanding of f numbers for a lens, I have read lots of articles which try to explain these numbers, I read that the f number is a dimensionless number, and am trying to understand the value of the numbers, I know that the lower the number the wider the aperture and when the lens is extended the light that goes through the aperture drops.
Do the numbers have any relationship from one lens to another? Is the f number a universal number? Using the same camera is the amount of light from a small lens set at f3, going to be the same amount of light let in by one of these massive lenses you see at sporting events that is also set at f3.
 
Hi Keith.
Yes f3 is the same on a 17mm lens as a 500mm lens.
 
I found this video helpful in regards to f-stop numbers.

[youtube]KmNIouLByJQ[/youtube]
 
The point is to ensure that the exposure is the same between different focal length lenses when used at the same aperture so although the aperture is actually a different size from lens to lens the exposure remains the same :D The f number does relate to the actual size of the aperture, focal length divided by f number = aperture size. So a 100mm lens at f10 has a 10mm aperture to let the light in and a 50mm lens at f10 has a 5mm aperture, etc...
 
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Digifrog said:
Do you have Flash* installed? Are you running the latest version of your web browser of choice?
Click to expand...

Fixed :thumbs:

*If you are using Linux, please also note:

"When Adobe announced ...that it plans to discontinue its standalone Flash Player for Linux, it noted that updated versions of Flash Player will subsequently be available to Linux users only through Google's Chrome browser.
...In the meantime, though, it's worth remembering that Flash Player is not the only way to view Flash content. In fact, as a reader recently reminded me, there are open source alternatives to Flash Player that offer Linux users another option."(Source)
 
Hi,
I have found the video clip and the examples really useful. Hypothetically speaking without doing the maths would the f-number be an infinite number, I know it’s not practical but say if I put a lens on my camera with the diameter of 500mm would they be an f-number for this?
 
Hi,
This is not the full explanation is it, my reading of it is my zoom lens is set at f3.5 and the lens is set at 70mm and the aperture or iris is fully open, if I extend my zoom lens to 300mm and the aperture or iris is still wide open but the f-number goes up because of the tunnel effect, so the aperture stays the same diameter but the f-number goes up.
Is the f-number an infinite number or until it becomes impractical?
 
It's not an infinite number, quite the opposite, as it reduces as the focal length increases.....It's a fraction, f/ not f.

Bob
 
Thanks Bob,
So you could go lower than f1 to what ever if it was practical?
 
Hi Keith. I'm pretty new to photography but I'm not too bad at mathematics.

You've said two things that suggest to me that your issue might be maths understanding.

Keith Butler said:
I read that the f number is a dimensionless number
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Keith Butler said:
f-number be an infinite number
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I think by "Dimensionless" you may mean that it is an abstract number based on two variables.

I think by "infinite number" you may mean "infinitely variable number".

As has been said, most recently by etaf, Aperture stop = Focal Length divided by Diameter of iris.

I think you're conflating iris diameter and aperture stop.

In the example you are giving aperture stop changes because focal length changes.

F-stop will equal infinity when the iris is closed.

I hope that's of help.
 
Keith Butler said:
Thanks Bob,
So you could go lower than f1 to what ever if it was practical?
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Lower than f/1 is possible (there are a few lenses) but it means that the apparent aperture diameter is larger than the focal length.

Bob
 
Garnett Hi,
The dimensionless bit was the word used by the encyclopaedia web site and I think they mean if the aperture was set at 20mm diameter you can get different f-numbers for this diameter with the same lens.
 
Keith,

Although others have correctly stated the facts, I'll have a go in different words.

The aperture is not a physical thing....ie, it's not diameter of the front element or the diameter of the diaphragm. The aperture diameter is the perceived diameter of the opening that you see when looking through the lens. This is affected by the diaphragm diameter and the magnification provided by any lens elements before the diaphragm (and afterwards if the design is such).

As the focal length of the lens increases the aperture would need to increase proportionally to maintain the same ratio. For example;
A 600mm lens needs an aperture of 150mm to achieve f/4 (600/150)
A 100mm lens needs an aperture of 25mm to achieve f/4 (100/25)

With a zoom lens, if the aperture diameter remains the same then it you have 75mm / 25mm =f/3 but 300mm / 25mm =f/12. Of course, f/12 would be quite useless in general terms but the increased "perceived" size of the aperture (magnification effects) keep the f number within usable parameters.

How's that?

Bob
 
Hi Bob,
That sounds pretty good thank you, with the more experienced photographers all the combinations of f-numbers, shutter speed and iso numbers will be like second nature but me being a novice it helps to understand what does what. Many thanks
 
thats also why zoom lenses which maintain the same aperture across the range are quite expensive
 
probably going to hit you with a few other terms here

for a 50mm focal length

now a diameter hole size of

12.5mm hole

50/12.5 = 4 so F4

1mm hole

50/1 = 50 so F50

0.25mm hole

50/.25 = 200 so F200

50mm hole

50/50 = 1 so F1

100mm hole

50/100 = 0.5 so F0.5

heres a lens at F1.1
http://www.luminous-landscape.com/reviews/lenses/50mm-nokton.shtml

you also need to think about Depth of Field ( DoF ) when thinking about apertures

theres a great online calculator to work out DoF for different focal lengths and apertures which also include the camera and sensor type
http://www.dofmaster.com/dofjs.html

the sensor is important a full frame camera - which means one that has the same size sensor as a film camera and so the focal length (field of view) is the same as a film camera
 
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i'm fairly familiar with this (ish) but I still find this interesting
however, one thing that may be relevant (or I may be wrong)
if you zoom in from 70 to 200 on a fixed aperture lens then you will still need to adjust the exposure as there is less ambient light ( in general) to enter the lens in the field of your view, unless you're zoom in on a light source of course!
please correct me if I'm wrong, cheers
 
DizMatt said:
....however, one thing that may be relevant (or I may be wrong)
if you zoom in from 70 to 200 on a fixed aperture lens then you will still need to adjust the exposure as there is less ambient light ( in general) to enter the lens in the field of your view, unless you're zoom in on a light source of course!
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If it's a fixed aperture then the aperture size will increase as you increase the FL and exposure remains the same (due to the maintaining the same FL and aperture relationship). The exposure may change if you zoom in on an area that has a different average luminosity than the initial framed image (it could increase or decrease dependent on the subject).
If you were to walk backwards as you increased the FL so as to maintain the same framed shot then the exposure would be the same.

Edit...I am assuming here that your term "fixed aperture" is actually meant to imply "constant aperture"....ie, f/4 at 24mm and also f/4 at 100mm for example.

Bob
 
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Garnett said:
I think by "Dimensionless" you may mean that it is an abstract number based on two variables.
I hope that's of help.
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No, he means a dimensionless number, because it's the ratio of a focal length in mm to a diameter in mm
There are plenty of dimensionless numbers in science and engineering; e.g. the Reynolds number in fluid flow, the drag coefficient of a car or the Mach no of a high speed object. Afull list here

http://en.wikipedia.org/wiki/Dimensionless_quantity
 
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Canon Bob said:
The exposure may change if you zoom in on an area that has a different average luminosity than the initial framed image (it could increase or decrease dependent on the subject)...
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I see what you're saying, but to clarify:
If the camera is set to fully manual there should be no change in exposure for a given area of an image.
If using a auto/semi auto mode the camera, as Canon Bob said above, will evaluate and set the exposure according to what is in the viewfinder.
For example a landscape at 30mm (Set @f. 8) could have some darker trees and lots of bright blue sky. The camera will pick an average* for the whole scene. Sky looks ok and the trees a bit dark.
Zoom in on a dark green bush at 200mm (again set @ f.8) and the camera will expose for just the bush and use a longer shutter speed, so the bush will look brighter than in the first image.

In theory lenses should be the same for any given F number. In practice there can be slight variations between lenses due to efficiancies and light loss in the lens but that's just going to confuse things :thumbs:
(* For simplicity, but does depend on metering settings.)
 
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mortimerhill said:
No, he means a dimensionless number, because it's the ratio of a focal length in mm to a diameter in mm
There are plenty of dimensionless numbers in science and engineering; e.g. the Reynolds number in fluid flow, the drag coefficient of a car or the Mach no of a high speed object. Afull list here

http://en.wikipedia.org/wiki/Dimensionless_quantity
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Yes, I read wikipedia too, but decided not to change my post since if you look at the examples you've cut and pasted, they're all abstract numbers based on two variables.

I think the problem here is that "aperture" has been used interchangeably with "aperture stop".
 
Hi everyone,
Thanks for all the advice and explanations they are very useful. I actually was out on the coast yesterday taking shots (in the rain and dark light), and found the limitations of my kit lenses, the max of f3.5 falls a bit short.
 
the max of f3.5 falls a bit short.
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depends if you also want DoF in which case you would need to increase the ISO
just by having a fast 1.8 /1.4 lens does not actually always allow for images to be taken , if a reasonable DoF required
 
etaf said:
thats also why zoom lenses which maintain the same aperture across the range are quite expensive
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At the risk of derailing the thread, here's a question that I've never thought about before but this seems an appropriate place to ask. If you take a 70-200 2.8 lens. So it can offer f/2.8 at 200mm so presumably has a max aperture of 71.4mm in diameter. Why can it not offer f/1 at 70mm?
 
Graham,
I am probably way out of my depth but is the lens spec 2.8 at 70mm or 200mm?
 
I am probably way out of my depth but is the lens spec 2.8 at 70mm or 200mm?
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should be 2.8 throughout

quite an expensive bit of kit though c£2k
 
Keith Butler said:
Graham,
I am probably way out of my depth but is the lens spec 2.8 at 70mm or 200mm?
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Just a hypothetical example but yeah there are loads of 70-200 lenses on the market that offer f2.8 throughout their zoom range. I could equally have asked the same about a 24-70 f2.8 or a 24-120 f4 or any other zoom lens that offers a constant max aperture throughout it's range. Just wondering what stops them from benefitting from a larger effective aperture when they're used at their shortest length.
 
gad-westy said:
At the risk of derailing the thread, here's a question that I've never thought about before but this seems an appropriate place to ask. If you take a 70-200 2.8 lens. So it can offer f/2.8 at 200mm so presumably has a max aperture of 71.4mm in diameter. Why can it not offer f/1 at 70mm?
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Two reasons....
The 71.4mm is not a physical dimension, it's the diameter that the "hole" through the lens appears when you look through it from the front.....this includes the magnification effects of the elements within. This magnification effect will change as the elements move to facilitate the change in focal length.
The second reason is not always present. The diaphragm may be linked to the zoom and close to keep the aparent aperture the same. Simply having the same aparent aperture without recourse to the optical formula (distortion etc) would be nigh on impossible to achieve.

Bob
 
gad-westy said:
Just a hypothetical example but yeah there are loads of 70-200 lenses on the market that offer f2.8 throughout their zoom range. I could equally have asked the same about a 24-70 f2.8 or a 24-120 f4 or any other zoom lens that offers a constant max aperture throughout it's range. Just wondering what stops them from benefitting from a larger effective aperture when they're used at their shortest length.
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See my post above (written whilst you were posting)

Bob
 
well to do F1 at 70mm would need to have 200mm at 200mm Focal length to keep the aperture across the range - which is what people would like
 
Canon Bob said:
Two reasons....
The 71.4mm is not a physical dimension, it's the diameter that the "hole" through the lens appears when you look through it from the front.....this includes the magnification effects of the elements within. This magnification effect will change as the elements move to facilitate the change in focal length.
The second reason is not always present. The diaphragm may be linked to the zoom and close to keep the aparent aperture the same. Simply having the same aparent aperture without recourse to the optical formula (distortion etc) would be nigh on impossible to achieve.

Bob
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Much appreciated. Crystal clear now.
 
etaf said:
well to do F1 at 70mm would need to have 200mm at 200mm Focal length to keep the aperture across the range - which is what people would like
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I think you're looking at it the other way around. I was saying that we already have f/2.8 at 200mm so why doesn't that translate to f/1 at 70mm? Rather than asking for an f/1 200mm lens. Though that would be nice wouldn't it? Wouldn't fancy hand holding it mind.
 
Canon Bob said:
If it's a fixed aperture then the aperture size will increase as you increase the FL and exposure remains the same (due to the maintaining the same FL and aperture relationship). The exposure may change if you zoom in on an area that has a different average luminosity than the initial framed image (it could increase or decrease dependent on the subject).
If you were to walk backwards as you increased the FL so as to maintain the same framed shot then the exposure would be the same.

Edit...I am assuming here that your term "fixed aperture" is actually meant to imply "constant aperture"....ie, f/4 at 24mm and also f/4 at 100mm for example.

Bob
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yes. I agree. In my experience when I zoom in, it always seems to be on a subject with lower luminosity than the surroundings as windows and other light sources are lost from the frame. Therefore I need to compensate for that...but I understand your point, well made.
 
Keith Butler said:
Hi Bob,
That sounds pretty good thank you, with the more experienced photographers all the combinations of f-numbers, shutter speed and iso numbers will be like second nature but me being a novice it helps to understand what does what. Many thanks
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I understand f numbers but reading all of that math made me a bit dizzy.lol. seriously, (imho) knowing the mechanics of f stop is great but understanding the effects is all that really matters. The minute I stopped trying to understand the the math and definitions and went out with a test target to see how different stops effected the dof was the minute that it all came together for me and has been clear sense. Although I did not read every comment here I read most of them and I don't recall any mention of dof, that's Odd isn't that the point...
 
shaylou said:
..... Although I did not read every comment here I read most of them and I don't recall any mention of dof, that's Odd isn't that the point...
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Without doubt DoF is the consideration in many shots but, alas, Keith's question was about the effects F-numbers have on exposure.

I think it's better to get an understanding of the separate parts before considering the whole.

Bob
 
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